Casing joints, liners, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be placed in a wellbore to stabilize a formation and protect a formation against high wellbore pressures (e.g., wellbore pressures that exceed a formation pressure) that could damage the formation. Casing joints are sections of steel pipe, which may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections are usually designed so that a seal is formed between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomer seal (e.g., an o-ring seal), a thread seal, a metal-to-metal seal formed proximate the connection, or similar seals known in the art.
In FIG. 1, a connection having a metal-to-metal seal at an intermediate position is shown. Such a connection is disclosed as an embodiment of U.S. Pat. No. 6,543,816 issued to Noel. That patent is incorporated herein by reference in its entirety. The connection includes a pin member 101 and a box member 102, each with a single tapered thread, 18 and 19 respectively. The pin thread 18 and the box thread 19 are interrupted at an intermediate position to form corresponding seal surfaces, which form a metal-to-metal seal 20 when a sufficient contact pressure exists therebetween. The metal-to-metal seal 20 shown in FIG. 1 is commonly referred to as a “mid-seal” due to its intermediate position, as opposed to other types of metal-to-metal seals located on the end of the pin member or the box member. To ensure that the desired contact pressure exists to form metal-to-metal seal 20, a positive stop is provided by pin nose 23 on the pin member 101 and shoulder 22 on the box member 102.
One type of thread commonly used to form a thread seal is a wedge thread. In FIG. 2, a connection having a wedge thread is shown. “Wedge threads” are characterized by threads, regardless of a particular thread form, that increase in width (i.e., axial distance between load flanks 225 and 226 and stab flanks 232 and 231) in opposite directions on the pin member 101 and box member 102. The rate at which the threads change in width along the connection is defined by a variable commonly known as a “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the threads to vary width along the connection. A detailed discussion of wedge ratios is provided in U.S. Pat. No. 6,206,436 issued to Mallis, and assigned to the assignee of the present invention. That patent is incorporated herein by reference in its entirety.
Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference. Continuing with FIG. 2, on the pin member 101, the pin thread crest 222 is narrow towards the distal end of the pin member 101 while the box thread crest 291 is wide. Moving along the axis 105 (from right to left), the pin thread crest 222 widens while the box thread crest 291 narrows. In FIG. 1, the thread surfaces are tapered, meaning that the pin thread 106 increases in diameter from beginning to end while the box thread 107 decreases in diameter in a complimentary manner. Having a thread taper improves the ability to stab the pin member 101 into the box member 102 and distributes stress in the connection.
Generally, thread seals are difficult to achieve with non-wedge threads having broad crests and roots, however, the same thread forms may have thread seals when used for wedge threads. Wedge threads do not have any particular thread form. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239 issued to Klementich, and incorporated herein by reference. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706 issued to Church, and incorporated herein by reference. Each of the above thread forms is considered to be a “trapped” thread form, meaning that at least a portion of the corresponding load flanks and/or corresponding stab flanks axially overlap. An open (i.e. not trapped) thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880 issued to Watts. The above thread forms are examples of thread forms that may be used for embodiments of the invention. Generally, open thread forms such as buttress or stub acme are not suitable for wedge threads because they would impart a large radial force on the box member. A generally square thread form, such as that disclosed by Watts, or a trapped thread form does not impart an outward radial force on the box member. Those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
For wedge threads, a thread seal is accomplished by the contact pressure caused by interference over at least a portion of the connection between the pin load flank 226 and the box load flank 225 and between the pin stab flank 232 and the box stab flank 231, which occurs when the connection is made-up. Close proximity or interference between the roots 292 and 221 and crests 222 and 291 completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Generally, higher pressure may be contained with increased interference between the roots and crests (“root/crest interference”) on the pin member 101 and the box member 102 and by increasing flank interference. This particular connection also includes a metal-to-metal seal that is accomplished by contact pressure between corresponding seal surfaces 103 and 104, respectively located on the pin member 101 and box member 102.
Wedge threads typically do not have a positive stop torque shoulder on the connection. For wedge threads that do not have a positive stop torque shoulder, the make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member varies more during make-up for a given torque range to be applied than for connections having a positive stop torque shoulder. As used herein, “make-up” refers to threading a pin member and a box member together. “Selected make-up” refers to threading the pin member and the box member together within a desired range of torque, or based on a relative position (axial or circumferential) of the pin member with the box member. For wedge threads that are designed to have both flank interference and root/crest interference at a selected make-up, both the flank interference and root/crest interference increase as the connection is made-up (i.e. increase in torque increases flank interference and root/crest interference). For wedge threads that are designed to have root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks and corresponding roots and crests come closer to each other (i.e. clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the make-up torque on the connection. Thus, a wedge thread may be able to thread-seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the make-up torque on the connection, however, this also increases stress on the connection during make-up, which could lead to failure during use.
Although wedge threads provide a thread seal in most cases, they are typically unable to seal gases. Most metal-to-metal seals, which are more capable of sealing gases, require relatively thick connections. To seal against both internal and external gas pressure, a connection could have a metal-to-metal seal on the pin nose for internal pressure and a metal-to-metal seal on the box face for external pressure, but this would increase the thickness of the connection. It would be desirable to obtain gas sealing capability against both internal and external pressure with a thin connection.